1. Field of the Invention
The invention is concerned with the stabilization of essentially saturated hydrocarbon polymeric materials against oxidative degradation and is particularly concerned with such stabilization under circumstances that the polymer is in intimate contact with copper or other metals known to accelerate the oxidative degradative process.
2. Description of the Prior Art
It is well recognized that saturated hydrocarbon polymeric compositions degrade under the influence of oxygen. This degradation which involves mechanical as well as electrical properties increases with increasing temperature and is, accordingly, known as thermal degradation. It is also well known that the degradative process is accelerated by the presence of copper and certain other active metals. The degradative process is believed initiated at the sites of tertiary carbon atoms on the polymer chain and, accordingly, is more pronounced, for example, in polypropylene which, by its nature, contains a tertiary carbon for every two carbons in the main chain, than in polyethylene which may contain only from 1/50 to 1/1000 tertiary carbons expressed as a fraction of the total number of carbon atoms in the polymer chain.
The thermal oxidative degradation process in polyolefins is considered to be autocatalytic, proceeding from the production of free radicals resulting from hydroperoxide production. Catalysis of the degradative process by copper or other metal ions is thought to proceed by the formation of unstable coordination complexes which facilitate free radical production.
Thermal degradative effects are minimized in such compositions generally by any of a class of hindered phenols or aryl amines which combine with free radicals to act as chain terminators. A large variety of such "thermal antioxidants" are commercially available and any of several provide adequate protection for many uses. Nevertheless, there is some continuing effort to develop new antioxidants, sometimes for economic reasons, sometimes to minimize effect on certain polymer properties, and, always, to still further extend lifetime.
An area of some continued activity concerns well engineered, long life expectancy polymeric products which are necessarily maintained in contact with copper or other active metals known to accelerate degradation. Perhaps the most prevalent category is that of primary insulation on copper wire (although similar effects are seen to a lesser extent where copper is replaced by other conductive metals, such as, aluminum). The effect of copper is lessened by any of several metal deactivators which somehow chemically combine with the metal ions. Oxamide, as well as some of its derivatives, accomplishes this end and has been used successfully in polymers including polypropylene and polyethylene. See 5, Polymer Engineering and Science 3 (1965). Another class of metal inhibitors is made up of the reaction products of an orthohydroxy aromatic carbonyl compound with an alkyl diamine. A more recently developed class of compounds, now in prevalent use in polyethylene, includes the reaction product of benzaldehyde and oxalyl dihydrazide. Such compounds, as represented by N,N'-dibenzal (oxalyl dihydrazide), are now in widespread use in polyethylene and ethylene propylene primary insulation. See U.S. Pat. No. 3,440,210. In general, where particularly long life is required, such protected compositions contain both a metal deactivator and a conventional thermal antioxidant.